Agriculture Reference
In-Depth Information
of the bioavailable fraction of an environmental contaminant into the same combined response.
Because it is an early response, it may, for certain substances at least, also reflect the effect of
contamination age or time on bioavailability (Reinecke and Reinecke 2003). It may be able to
differentiate whether equilibrium has been attained in the substrate matrix. It could also (under
similar conditions of pH, soil texture, etc.) reveal differences in the bioavailability of a substance
in a spiked substrate compared with that in a contaminated field sample containing exactly the
same concentration of toxicant but with a long exposure history. Because the bioavailability of a
substance can be dynamic and variable, simple biomarker responses could provide an ideal method
for the regular assessment of changes, provided that response sensitivity is maintained and that
practical feasibility of methodology and execution can be achieved.
Contaminated soils usually contain mixtures of toxicants (Weltje 1998), causing multiple
exposures, which in any case make the causal linking of a response to one toxicant in the mixture
very difficult, if not impossible. The nonspecificity of some biomarkers that is generally perceived
as a drawback in ecological risk identification may therefore still be utilized. Although the relative
contribution of the different factors to the stress response cannot be singled out, it may still provide
a practical and realistic tool for purposes of assessing the stresses caused by the bioavailability of
a chemical, in conjunction with existing confounding factors, under field conditions. After all, if
the protection of biodiversity is kept in mind, the mere establishment of stressful conditions,
irrespective of their nature or origin, at an early stage is of primary importance.
The idea of using biomarkers for the more accurate quantification of exposure to contaminants
is gaining support (Kammenga et al. 2000; Spurgeon et al. 2000). Given the complexities involved
in dealing with the various parameters that can affect the bioavailability of contaminants in soils
and the difficulties in trying to incorporate them into a practical risk assessment procedure, alter-
native approaches are needed. Therefore, it is not surprising that a trimming of risk assessment
procedures to a more manageable scale is sought by many regulatory authorities. This can be
achieved by acknowledging the biological nature of the bioavailability concept and using a realistic
biological tool such as earthworm biomarkers to assess toxicant bioavailability or to validate the
estimated toxicant bioavailability values obtained by chemical fractionation methods such as the
PBASE (Basta and Gradwohl 2000).
The Earthworm Ecotoxicology Workshop held in Denmark in 2001 concluded that, for a
biomarker or a battery of biomarkers to be useful and applicable under field conditions, they should
comply with some basic requirements:
1.
Chemicals
: Information on specificity is needed about the range of hazards (including
chemicals) that elicit a biomarker response; that is, biomarkers responding to a broad
range of chemicals or other factors can be used in monitoring or screening studies, and
biomarkers responding to a narrow range of compounds can be used in the case of known
pollutants. Therefore, the mechanistic connections of the biomarker response to the
desired end point must be known.
2.
Linking to higher levels
: Because the ultimate aim is to validate a marker for regular
toxicity test use, there should be a correlation (linkage) between the biochemical or other
marker response and deleterious changes to the population or community. The Denmark
workshop recommended that biomarkers should be used that are connected to the life
cycle parameters of earthworms apart from their possible linkage to pure compounds. It
is therefore important to test biomarkers in field experiments with known earthworm
population changes to see if the biomarkers possess predictive abilities. For example, is
the biomarker responding even before a population response occurs? This was seen in
the study of Maboeta et al. (2002), in which the neutral red retention response preceded
declines in the earthworm populations.
3.
Inherent variation and temporal aspects
: For a biomarker response to be useful and
reliable in the field, it should ideally have a low inherent variability with a limited, or
